In a vehicle engine, in order to prevent the speed from falling due to load changes when the engine is running idle, a type of control is known where the idle rotation speed is feedback controlled so that the real rotation speed is identical to a target rotation speed. However when the idle rotation speed is fixed at a predetermined level, the engine rotation speed (idle rotation speed) may be too high when the brake is operated in order to decelerate, and the driver experiences discomfort during the deceleration.
In this regard, Tokkai Sho 62-233429 published by the Japanese Patent Office in 1987 proposes an engine idle rotation controller which, when the brake is operated, drops the target rotation speed as compared to the case when the brake is not operated. Herein the operation of the brake refers to the depression of a brake pedal or operation of a parking brake.
However, if the brake pedal is depressed and released as when, for example, the vehicle is being garaged or is following a vehicle in front of it in a traffic jam, the vehicle moves at a low speed. In the case of the aforesaid controller, even under these conditions, the target rotation speed decreases every time the brake is depressed. When the brake pedal is next released, therefore, creep force of the vehicle is insufficient so that either the vehicle does not start or starts only very sluggishly. As a result, the accelerator pedal must again be depressed to start the vehicle, hence the accelerator pedal must be operated frequently. The creep force refers to the force tending to move the vehicle when the automatic transmission is within traveling range and the accelerator pedal is not at all depressed. Herein traveling range refers to a range other than the parking and neutral ranges.
However, when the driving force is set so that sufficient creep force is obtained even when the target rotation speed has decreased, the creep force is then excessive when the automatic transmission is in traveling range and the brake is depressed to prevent the vehicle from moving.
The above case occurs frequently especially when the vehicle is in a short stop or waiting to start on a crossing, so the brake pedal must be strongly depressed to suppress the creep force.
Further in the aforesaid controller, the target rotation speed is decreased without considering the situation when the engine is warming up or the air conditioner is operating. When the engine is cold or the air conditioner is operating, the idle rotation speed easily tends to fall, so decreasing the target rotation speed under these conditions is undesirable from the viewpoint of preventing speed fluctuations.